An image forming apparatus that can perform color copying includes plural photoconductive drums corresponding to yellow (Y), magenta (M), cyan (C), and black (K), respectively, and also includes an exposing unit, a developing unit, a transfer belt, plural primary transfer rollers, plural secondary transfer rollers, and a heat roller.
The exposing unit exposes the surfaces of the photoconductive drums with laser beams to thereby form electrostatic latent images on the surfaces of the photoconductive drums, respectively. The developing units develop the electrostatic latent images formed on the surfaces of the photoconductive drums with yellow, magenta, cyan, and black developers to visualize the electrostatic latent images, respectively. The transfer belt rotates while coming into contact with the surfaces of the respective photoconductive drums. The primary transfer rollers press the rotating transfer belt against the surfaces of the photoconductive drums and transfer visible images on the photoconductive drums onto the transfer belt. The secondary transfer rollers transfer the visible images of the respective colors, which are transferred onto the transfer belt, onto a paper sheet. The heat roller applies heat to the paper sheet having the visible images transferred thereon to fix the visible images on the paper sheet. Printing of an image is completed by the fixing of the visible images.
After transferring the visible images of the respective colors onto the paper sheet, the transfer belt continues the rotation until developers remaining thereon are removed by a cleaner. However, when the transfer belt continues the rotation while coming into contact with the photoconductive drums, the surfaces of the photoconductive drums are worn and the durable life of the photoconductive drums is reduced.
Therefore, after the visible images on the transfer belt are transferred onto the paper sheet, the transfer rollers move to the opposite side of the transfer belt. The transfer belt is separated from the photoconductive drums by the movement of the transfer rollers. After the developers remaining on the transfer belt are removed by the cleaner, the transfer rollers move to the transfer belt side. The transfer belt is brought into contact with the photoconductive drums again by the movement of the transfer rollers.
In such an image forming apparatus, the density of an image printed on the paper sheet may deviate from intrinsic appropriate density, i.e., so-called absolute density due to contact failure of the transfer belt with respective photoconductive drums.
In order to prevent such deviation of the density, in an image forming apparatus disclosed in JP-A-2001-66835, patches are provided at both ends in main scanning for optically scanning an image of an original, the patches are scanned by the main scanning, a difference between densities of images obtained by the scanning is calculated, reference density is calculated from the calculated density difference, and an exposure amount on photoconductive drums is corrected on the basis of the calculated reference density.